Short-wavelength radiation source with multisectional collector module and method of collecting radiation

ABSTRACT

A radiation source contains a collector module comprising an optical collector, positioned in a vacuum chamber with an emitting plasma, further comprising a means for debris mitigation which include at least two casings arranged to output debris-free homocentric beams of the short-wavelength radiation, coming to the optical collector preferably consisting of several identical mirrors. Outside each casing there are permanent magnets that create a magnetic field inside the casings to mitigate charged fraction of debris particles and provide the debris-free homocentric beams of short-wavelength radiation. Other debris mitigating techniques are additionally used. Preferably the plasma is laser-produced plasma of a liquid metal target supplied by a rotating target assembly to a focus area of a laser beam. The technical result of the invention is the creation of high-powerful high-brightness debris-free sources of short-wavelength radiation with large, preferably more than 0.25 sr, collection solid angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation-in-part of U.S. patentapplication Ser. No. 16/773,240 filed Jan. 27, 2020, which in turn is aContinuation-in-part of U.S. patent application Ser. No. 16/535,404,filed on Aug. 8, 2019, which in turn is a Continuation-in-part of U.S.patent application Ser. No. 16/103,243, filed on Aug. 14, 2018, withpriority to Russian patent application RU2017141042 filed Nov. 24, 2017,the present application also claims priority to Russian patentapplication RU2020129329 filed Sep. 4, 2020, all of which areincorporated herein by reference in their entireties.

FIELD OF INVENTION

The invention relates to high-brightness radiation sources designed togenerate soft X-ray, extreme ultraviolet (EUV) and vacuum ultraviolet(VUV) radiation at wavelengths of approximately 0.4 to 200 nm and methodof collecting radiation, which provide highly effective debrismitigation in large collection angle to ensure the long-term operationof the high-power light source and its integrated equipment.

BACKGROUND OF INVENTION

Sources of radiation of soft X-ray, extreme ultraviolet (EUV) and vacuumultraviolet (VUV) ranges of high intensity are used in many fields: formicroscopy, biomedical and medical diagnostics, material testing,analysis of nanostructures, in atomic physics, and lithography.

A plasma effectively emitting in the soft X-ray range (0.4-10 nm), EUV(10-20 nm) and VUV (20-120 nm) ranges can be obtained both by focusingthe radiation of high-power lasers on the target and in the discharge.

From international patent application PCT/EP2013/061941, published undernumber WO 2014/001071 on Aug. 22, 2013 a laser-produced plasma (LPP) EUVlight source with collector module comprising: a collector forcollecting radiation generated by a radiation generating plasma, and fordirecting the generated radiation; further comprising a means forsuppression of infrared laser radiation in the beam of plasma radiationis known.

LPP EUV light source is characterized by a high brightness. However,there is a problem of protecting the optical collector from debris toensure a long lifetime of the LPP EUV light source

The debris, generated as a by-product of the plasma during the radiationsource operation, can be in the form of high-energy ions, neutral atomsor vapors and clusters of the plasma fuel material. Debris particlesdegrade the collector optics, which can consist of one or severalcollector mirrors located near the radiation source. In addition to thefact that microdroplets and particles deposited on the collector mirrorreduce its reflection coefficient, high-velocity particles can damagethe collector mirror and, possibly, other parts of the optical systemlocated behind the collector mirror. This makes it urgent to developdebris-free high-brightness sources of short-wavelength radiation.

From the international patent application PCT/RU2012/000701, publishedunder number WO/2013/122505 on Aug. 22, 2013, a laser-triggereddischarge plasma EUV light source is known. The focused laser beam isdirected to one of the electrodes so that the laser-triggered dischargehas an asymmetric, predominantly curved, banana-like shape. Theintrinsic magnetic field of such discharge has a gradient thatdetermines the predominant motion of the discharge plasma to the regionof a weaker magnetic field. The direction of plasma flow issignificantly different from the direction to the optical collector. Toobtain a high radiation power, the discharge is produced at a high pulserepetition rate. The invention provides a simple and highly effectivemitigation of the charged particles.

However, suppression of the neutral particles and clusters requires theuse of more sophisticated debris mitigation techniques.

The light generation in the soft X-ray, EUV and VUV ranges is mosteffective with the use of laser-produced plasma. The development LPPradiation sources in recent years has been largely stimulated by thedevelopment of projection extreme ultraviolet lithography forhigh-volume manufacturing of integrated circuits (ICs) with 7-nm nodeand below.

A debris mitigation technique based on the use of auxiliary plasmagenerated along the path of a short wavelength radiation beam in aspecially injected gas is disclosed in U.S. Pat. No. 9,268,031 publishedon Feb. 23, 2016. Debris that acquire an electric charge as a result ofexposure to the auxiliary plasma are then deflected by a pulsed electricfield. The method is effective for protection optical collector againstion/vapor fraction of debris, for example, in sources using xenon asplasma fuel.

However, in sources using metals as plasma forming material, the mainthreat to the elements of the optical collector is the micro-dropletfraction of debris particles, against which this method is powerless.

From U.S. Pat. No. 8,519,366, published on Aug. 27, 2013, a debrismitigation method in LPP EUV radiation source using Sn droplet targetsis known. The method involves using a magnetic mitigation of chargedfraction of debris particles. Along with this, the debris techniqueincludes a foil traps and ports for supplying the protective flows ofbuffer gas, which provides a sufficiently effective trapping of neutralatoms and clusters of the liquid metal target material.

However, additional, rather complex means are required for mitigatingmicro-droplet fraction of debris particles.

The method for debris mitigating, known from U.S. Pat. No. 7,302,043,published on Nov. 27, 2007, is partially devoid of this drawback. Itprovides for the use of a fast-rotating shutter capable of transmittingshort wavelength radiation through at least one opening during onerotation period and preventing the passage of debris during anotherrotation period of the shutter.

However, the use of such means for debris mitigation in a compactradiation source is technically too difficult to implement.

This drawback is largely devoid of shortwave radiation sources knownfrom U.S. Pat. No. 10,638,588, published on Apr. 28, 2020, U.S. Pat. No.10,588,210, published on Mar. 10, 2020, and US patent application20200163197, published on May 5, 2020, which are incorporated into thisdescription by reference in their entirety. The sources disclosed inthese patent documents contain a vacuum chamber with a rotating targetassembly delivering a target in the form of a molten metal layer to theinteraction zone with a focused laser beam. The complex of means fordebris mitigation includes the rotation of the target with a high linearvelocity, more than 80 m/s. To suppress the ion/vapor fraction ofdebris, the use of foil traps, magnetic fields and directional flows ofprotective buffer gas is provided. In embodiments of the radiationsource, a replaceable membrane of carbon nanotubes (CNT membrane) isinstalled in the path of the short-wavelength radiation beam. Also thedebris shield, surrounding region of the emitting plasma, is fixedlyinstalled providing entering the laser beam into the region of thepulsed emitting plasma and exiting a short-wavelength radiation beamfrom it. It is also proposed to use a laser prepulse to suppress theionic fraction of debris. Another proposed debris mitigation mechanismis to use a high repetition rate of laser pulses, for example, on theorder of 1 MHz, in order to ensure the evaporation of microdroplets upto 0.1 μm in size resulting from the previous laser pulse by radiationand plasma of the subsequent pulse.

These methods have a sufficiently high efficiency of debris mitigation,however, they are intended for relatively small spatial angles ofcollection of short-wavelength plasma radiation, as a result, theaverage power in the short-wavelength radiation beam turns out to beinsufficient for a number of applications.

SUMMARY

Accordingly, there is a need to eliminate at least some of the drawbacksmentioned above. In particular, there is a need for improved lightsources that are compact, high-powerful with large collecting angle andprovide substantially complete debris mitigation in the path of theoutput beam of short-wavelength radiation.

This invention is aimed at solving a technical problem associated with amultiple increase in the average power of pure high-brightness sourcesof soft X-ray, EUV and VUV radiation while ensuring their commercialavailability and economic operation.

The technical result of the invention is the creation of high-powerfulhigh-brightness sources of short-wavelength radiation with highlyeffective debris mitigation in a beam of short-wavelength radiationpropagating in a large, preferably more than 0.25 sr, solid angle.

Achievement of the purpose is possible by means of a plasmashort-wavelength radiation source with a collector module comprising anoptical collector, positioned in a vacuum chamber with plasma emitting ashort-wavelength radiation, further comprising a means for debrismitigation on a path of the short-wavelength radiation to the opticalcollector.

The source is characterized the means for debris mitigation includes atleast two casings arranged to output debris-free homocentric beams ofthe short-wavelength radiation coming to the optical collector, andoutside each casing there are permanent magnets that create a magneticfield inside the casings, and a magnetic field formed by the permanentmagnets removes charged fraction of debris particles from thehomocentric beams to provide the debris-free homocentric beams.

Preferably, an outer surface of each casing contains two first facesextended substantially parallel to a direction of short-wavelengthradiation propagation from the plasma and parallel to a vertical or toanother chosen direction.

Preferably, each casing includes two second faces extended substantiallyparallel to the direction of short-wavelength radiation propagation fromthe plasma and substantially perpendicular to the two first faces of thecasing.

The embodiment of invention an area of first faces of each casing isgreater than an area of the rest of the casing surface, and thepermanent magnets are substantially in contact with the first faces ofeach casing.

The embodiment of invention an area of the first faces of each casing isless than an area of the rest of the surface of the casing, and thepermanent magnets are located on the surface of the casings outsidetheir first faces.

The embodiment of invention an angle between the two first faces of eachcasing is less than 30 degrees.

The embodiment of invention an angle between adjacent faces of the twoadjacent casings is from 3 to 10 degrees.

In the embodiment of invention the permanent magnets, located on a mostdistant from each other parts of the most distant from each othercasings, are connected by a magnetic core.

In the embodiment of invention the optical collector contains severalmirrors installed in the path of each of the debris-free homocentricbeams of the short-wavelength radiation.

Preferably, a reflecting surface of all mirrors form a spheroid, in onefocus of which is the plasma and in another focus is a focal point ofall mirrors of an optical collector.

Preferably, the means for debris mitigation include membranes based oncarbon nanotubes (CNT) installed between each casing and the opticalcollector in paths of the beams of short-wavelength radiation.

In the embodiment of invention the means for debris mitigation includeprotective gas flows, directed inside each casing into the plasma, whileeach CNT membrane simultaneously serves as a casing window for an exitof the debris-free homocentric beam of the short-wavelength radiationand a gas shutter preventing an exit of the protective gas through it.

In the embodiment of invention the permanent magnets are located alongan entire length of the casings.

Preferably, the means for debris mitigation include foil plates placedin each of the casings and oriented in radial directions with respect tothe plasma, substantially perpendicular to magnetic field lines.

In the embodiment of invention, the plasma can be selected from a groupconsisting of: laser-produced plasma, z-pinch plasma, plasma focus,discharge-produced plasma, laser-initiated discharge-produced plasma.

Preferably, the plasma is laser-produced plasma of a liquid metal targetsupplied by a rotating target assembly to a focus area of a laser beam.

Preferably, the target is a molten metal layer, formed by centrifugalforce on a facing to an axis of rotation surface of an annular groove,implemented in the rotating target assembly.

In another aspect, the invention relates to method of collectingradiation comprising: collecting by an optical collector radiationemitted by plasma at a plasma formation location, and directing at leasta portion of the radiation to a focal point, wherein the emitted byplasma radiation is guided through at least two casings equipped bymeans for debris mitigating and arranged to form debris-free homocentricbeams of the short-wave radiation coming out of casings to the opticalcollector.

Preferably, outside each casing there are permanent magnets that createa magnetic field inside the casings, and a magnetic field formed by thepermanent magnets mitigates charged fraction of debris particles andother debris mitigation techniques, including protective gas flow, foiltrap, CNT membrane are also used in each casing to provide thedebris-free homocentric beams.

Preferably, the optical collector contains several mirrors installed inthe path of each of the debris-free homocentric beams and a reflectingsurfaces of all mirrors lie on the surface of the ellipsoid or modifiedellipsoid, in one focus of which is the plasma, and in another focus isa focal point of all mirrors of an optical collector.

The above-mentioned and other objectives, advantages and features ofthis invention will be made more evident in the following non-limitingdescription of its embodiments, provided as an example with reference toattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The essence of the invention is illustrated by drawings, in which:

FIG. 1, FIG. 2—schematic diagrams of a short-wavelength radiation sourcewith multisectional collector module in accordance with the presentinvention,

FIG. 3, FIG. 4—schematic diagrams of a laser-produced plasma radiationsource with a rotating target assembly.

These drawings do not cover and, moreover, do not limit the entire scopeof the options for implementing this technical solution, but representonly illustrative material of a particular case of its implementation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an example of invention embodiment illustrated at variousscales in FIG. 1, a plasma radiation source comprises a vacuum chamber 1with a region of pulsed high-temperature plasma 2 emittingshort-wavelength radiation. As a by-product, the debris particles, whichinclude vapors, ions and clusters of the plasma-forming material, aregenerated in plasma region. The plasma radiation source furthercomprises a collector module consisting of an optical collector 3 and ofmeans 4 for debris mitigation placed on the path of the short-wavelengthradiation beam 5 directed from plasma 2 to the optical collector 3.Optical collector redirects the short-wavelength radiation to anintermediate focus and then to the optical system operating withshort-wavelength radiation.

In accordance with the invention, the means 4 for debris mitigationincludes at least two casings 6 arranged to output debris-freehomocentric beams 7 of the short-wavelength radiation coming to theoptical collector 3, preferably consisting of several mirrors 8.Characteristic plasma size is of about 0.1 mm (measured as the FWHM ofthe free electron density or the FWHM of the brightness profile of thelight emitting plasma region) therefore, the plasma radiation source canbe considered quasi-point, and radiation beams, coming out from it,homocentric.

Outside each casing 6 there are permanent magnets 9 that create amagnetic field inside the casings 6, and a magnetic field formed by thepermanent magnets 9 removes charged fraction of debris particles fromthe homocentric beams 7 to provide the debris-free homocentric beams.

The outer surface of each casing 6 contains two first faces 10 extendedsubstantially parallel to a direction of short-wavelength radiationpropagation from the plasma 2 and parallel to a vertical or to anotherchosen direction.

Outside each casing 6 are permanent magnets 9, which create a magneticfield inside the casings 6, the magnetic induction vectors of which aredirected substantially perpendicular to the optical axis of the casings.

Preferably, the permanent magnets 9 are located along an entire lengthof the casings 6.

In contrast to the known solutions, the means 4 for debris mitigation inaccordance with the present invention are a multi-section system thatallows to significantly increase the solid angle of collection ofshort-wavelength plasma radiation, while maintaining the high effectivedebris mitigation. An increase in the collection solid angle makes itpossible to significantly (several times) increase the collected powerof short-wavelength radiation and thereby increase the efficiency ofusing of such type radiation sources in almost all areas ofapplications.

In single-section systems, a simple increase in the transversedimensions of the housing leads to a sharp decrease in the effectivenessof the magnetic protection against charged particles. This is due to thefact that the larger the size of the casing along the lines of force ofthe magnetic field, the lower the values of the magnetic induction inthe volume of the casing, which leads to a decrease in the transversevelocity of charged particles propagating through the casing from theregion of the plasma emitting the short-wavelength 3 to the collectormirror 8. Thus, during the flight of the section, the particles cannotdeflect a sufficient distance to avoid hitting the mirror. Experimentshave shown that for the effective operation of the magnetic protection,it is necessary that the values of the magnetic induction in the centerof the casing at a distance of about 40 mm from the region of the plasmaemitting the short-wavelength were not less than 0.5 T. It has also beenexperimentally established that the flat angle between the sides of thecasing, on which the magnets are located, should not exceed 30 degrees.

Thus, the use of a multisection debris mitigation system, in which theplane angle between the faces of the casing does not exceed 30 degrees,makes it possible to create in each casing a constant magnetic field ofsufficient magnitude for high effective magnetic mitigation of chargedparticles.

In accordance with preferred embodiment of the invention, the permanentmagnets 9, located on a most distant from each other first faces 10 ofthe most distant from each other casings 6, are connected by a magneticcore 11. The magnetic core 11, preferably made of magnetically softsteel, makes it possible to reduce the loss of the magnetic fieldbecause of scattering by concentrating it in the magnetic core, andthereby increase it in the volume of each casing, increasing theefficiency of magnetic debris mitigation.

In an embodiment of the invention, each casing 6 includes two secondfaces 12 extended substantially parallel to the direction ofshort-wavelength radiation propagation from the plasma 2 andsubstantially perpendicular to the two first faces 10 of the casing.

The orientation of the first and second faces 10, 12 in the radialdirections with respect to the plasma 2 provides high geometrictransparency of the multisectional debris mitigation system. The samepurpose is served by the fact that in embodiments of the invention theangle between adjacent faces of the two adjacent casings 6 is in therange from 3 to 10 degrees.

In preferred embodiments of the invention, area of first faces 10 ofeach casing 6 is greater than an area of the rest of the casing 6surface, and the permanent magnets 9 are substantially in contact withthe first faces 10 of each casing 6.

In another embodiment (not shown) the area of the first faces 10 of eachcasing 6 can be less than an area of the rest of the surface of thecasing, and the permanent magnets 9 can be located on the surface of thecasings 6 outside their first faces 10, for example, on large secondfaces 12 of each casing 6.

The means 4 for debris mitigation preferably include membranes 13 fromcarbon nano tubes installed between each casing 6 and the mirror 8 ofthe optical collector 3 in the paths of the homocentric beams 7. TheCNT—membranes preferably have a thickness in the range of 20 to 100 nm,which ensures their high strength and high transparency in the range ofwavelengths shorter than 20 nm. So CNT membranes 13 provide the exit ofthe homocentric beams 7 due to their high transparency in the wavelengthrange shorter than 20 nm. At the same time, the CNT membranes 13 preventthe passage of debris particles through them, providing debris -freehomocentric beams 7 of short-wavelength radiation.

Along with this, the means for debris mitigation include protective gasflows, directed inside each casing 6 into the plasma, while each CNTmembrane 13 simultaneously serves as a casing window for an exit of thedebris-free homocentric beam 7 of the short-wavelength radiation and agas shutter preventing an exit of the protective gas through it.

Providing an average vacuum in the casings at a protective gas pressureof about 20 Pa allows to increase the number of collisions between gasmolecules and debris particles scattered from the plasma region and,thereby deflecting them from rectilinear motion. At the same time, theuse of a CNT membrane as a gas seal allows the use of increased pressureonly within the casings, and not along the entire path of propagation ofhomocentric beams 7 to the consumer optics. This reduces the loss ofshort-wavelength radiation due to absorption in the gas.

To obtain radiation in the wavelength range of more than 20 nm, CNTmembranes 13 are not used, since their transparency in the indicatedrange sharply decreases with increasing radiation wavelength.

In the preferred embodiment shown in FIG. 2, the optical collector 3contains several mirrors 8, while the reflecting surface of all mirrorsbelongs to the ellipsoid of revolution or in other words spheroid 15, inone focus of which is the region of pulsed emitting plasma 2, and in theother—the focus point 16 of the mirrors 8 of the optical collector 3.The production of such mirrors is very expensive, since the roughness ofthe collector mirror substrates is only 0.2-0.3 nm, and the cost of suchmirrors, especially with an aspherical profile, grows with an increasein their size according to a law 2-3 times stronger than an increase inarea. So, the use of several identical mirrors 8 significantly reducesthe cost of the optical collector.

The pulsed emitting plasma can be selected from the group consisting of:laser-produced plasma, z-pinch plasma, plasma focus, discharge-producedplasma, laser-triggered discharge plasma.

In a preferred embodiment, the pulsed high temperature plasma is thelaser plasma of a liquid metal target material delivered by a rotatingtarget assembly to the focusing region of the laser beam, as detailed inPatent Application 20200163197 published on May 21, 2020 which isincorporated herein by reference in its entireties.

In accordance with the preferred embodiment of the invention shownschematically in FIG. 3, the target 17 is a molten metal layer formed bycentrifugal force on the surface of the annular groove 19 of therotating target assembly 20 facing the axis of rotation 18. An isometricview of a preferred embodiment of the invention is shown schematicallyin FIG. 4.

The operation of the high brightness short-wavelength radiation sourcein the preferred embodiment using the laser-produced plasma of theliquid metal target illustrated in FIG. 3 and FIG. 4 is carried out asfollows. Vacuum chamber 1 is pumped out by an oil-free pumping system toa pressure below 10⁻⁵ . . . 10⁻⁸ mbar, removing gaseous components suchas nitrogen, oxygen, carbon, etc., capable of interacting with theliquid metal target material.

Target 17, the material of which belongs to the group of non-toxiclow-melting metals, including Sn, Li, In, Ga, Pb, Bi, Zn and theiralloys, is delivered by a rotating target assembly into the interactionzone with a focused laser beam 21. The target is exposed to a focusedpulsed laser beam 21 with a high pulse repetition rate in the range from1 kHz to 1 MHz. Depending on the target material and the laser powerdensity on the target, short-wavelength radiation of the laser plasma isgenerated in the soft X-ray and/or EUV and/or VUV spectral ranges.

The beam 5 of short-wave radiation emitted by the plasma 2, passingthrough the casings 6 and preferably through the CNT membranes 13, isconverted into debris-free homocentric beams directed to the mirrors 8of the optical collector 3. Herewith permanent magnets 9, FIG. 4, createa constant magnetic field, preferably directed perpendicular to the axisof the homocentric beams. Under the action of the Lorentz force, chargedfractions of debris particles (mainly ions) deviate from rectilinearmotion along the axes of the homocentric beams 7, colliding either withthe inner walls of the casings 6 or with plates 22 specially placed inthe casings, FIG. 3, are trapped by them. Plates 22 mounted in casings 6are directed radially to the plasma 2 and preferably perpendicular tothe lines of magnetic field created by magnets 9. Plates 22 allow moreefficient capture of high-speed charged particles, because the higherthe speed of the particles, the smaller the transverse distance theydeflect under the influence of the magnetic field. Along with thisprotective gas flows prevent the movement of the ion/vapor fraction ofdebris particles, depositing them on the walls of the casings 6 andplates 22, protecting the CNT membranes 13 from debris. Due to theirhigh transparency in the wavelength range shorter than 20 nm, CNTmembranes provide the exit of a short-wavelength beam to the mirrors 8of the optical collector 3. At the same time, CNT membranes 13 preventthe passage of debris through them, providing reliable protection foreach mirror 8. Additionally, effective debris mitigation in the casings6 is ensured by organizing directed flows of protective gas suppliedthrough the gas inlets 14. Shielding gas streams protect CNT membranes13 from ion/vapor fraction of debris, increasing their service life.

Similar means for debris mitigation are also used along the path of thelaser beam 21.

Above-described devices realize particular embodiments of the presentinvention relating in one of its aspects to a method of collectingradiation. The method comprises collecting by an optical collector 3short-wavelength radiation emitted by plasma 2 at a plasma formationlocation, and directing at least a portion of the radiation to a focalpoint 16, FIG. 2. The emitted by plasma 2 radiation beam 5 is guidedthrough at least two casings 6 integrated with means 4 for debrismitigating 4 and arranged to form debris-free homocentric beams 7 of theshort-wave radiation coming out of casings 6 to the optical collector 3.

Outside each casing permanent magnets 9 creating a magnetic field insidethe casings 6 are used for mitigation charged fraction of debrisparticles and other debris mitigation techniques, including protectivegas flow, foil trap, CNT membrane are also used in each casing toprovide the debris-free homocentric beams 7.

The optical collector 3 preferably contains several mirrors 8 installedin the path of each of the debris-free homocentric beams 7 and areflecting surfaces of all mirrors lie on the surface of the ellipsoid15 or modified ellipsoid, in one focus of which is the plasma 2, and inanother focus 16 is a focal point of all mirrors 8 of an opticalcollector 3. The modified ellipsoid shape may be used for providingimproved intensity uniformity of collected radiation in the far fieldcompared with a perfect ellipsoid shape.

Thus, the present invention makes it possible to create debris-free,powerful, high-brightness sources of soft X-ray, EUV and VUV radiationwith a long lifetime and ease of use.

INDUSTRIAL APPLICABILITY

The proposed devices are intended for a number of applications,including microscopy, materials science, X-ray diagnostics of materials,biomedical and medical diagnostics, inspection of nano- andmicrostructures, including actinic mask defect inspection for EUVlithography.

11

What is claimed is:
 1. A plasma short-wavelength radiation source with a collector module, comprising: an optical collector (3), positioned in a vacuum chamber (1) with a plasma (2) emitting a short-wavelength radiation , further comprising a means (4) for debris mitigation on a path of the short-wavelength radiation to the optical collector (3), wherein the means (4) for debris mitigation include at least two casings (6) arranged to output debris-free homocentric beams (7) of the short-wavelength radiation coming to the optical collector (3), and outside each casing (6) there are permanent magnets (9) that create a magnetic field inside the casings (6), and a magnetic field formed by the permanent magnets (9) removes charged fraction of debris particles from the homocentric beams (7) to provide the debris-free homocentric beams.
 2. The source according to claim 1, wherein an outer surface of each casing contains two first faces (10) extended substantially parallel to a direction of short-wavelength radiation propagation from the plasma (2) and optionally the two first faces are parallel to a vertical.
 3. The source according to claim 2, wherein each casing (6) includes two second faces (12) extended substantially parallel to the direction of short-wavelength radiation propagation from the plasma (2) and substantially perpendicular to the two first faces (10) of the casing.
 4. The source according to claim 2, wherein an area of first faces (10) of each casing (6) is greater than an area of the rest of the casing (6) surface, and the permanent magnets (9) are substantially in contact with the first faces (10) of each casing (6).
 5. The source according to claim 2, wherein an area of the first faces (10) of each casing (6) is less than an area of the rest of the surface of the casing (6), and the permanent magnets (9) are located on the surface of the casings (6) outside their first faces (10).
 6. The source according to claim 2, wherein an angle between the two first faces (10) of each casing (6) is less than 30 degrees.
 7. The source according to claim 2, wherein an angle between adjacent faces of the two adjacent casings (6) is from 3 to 10 degrees.
 8. The source according to claim 1, wherein the permanent magnets (9), located on a most distant from each other parts of the most distant from each other casings (6), are connected by a magnetic core (11).
 9. The source according to claim 1, wherein the optical collector (3) contains several mirrors (8) installed in the path of each of the debris-free homocentric beams (7).
 10. The source according to claim 9, wherein a reflecting surface of all mirrors (8) form an spheroid (15), in one focus of which is the plasma (2), and in another focus (16) is a focal point of all mirrors of an optical collector.
 11. The source according to claim 1, wherein the means (4) for debris mitigation include membranes (13) based on carbon nanotubes (CNT) installed between each casing (6) and the optical collector (3).
 12. The source according to claim 11, wherein the means (4) for debris mitigation include protective gas flows, directed inside each casing (6) into the plasma, while each CNT membrane (13) simultaneously serves as a casing window for an exit of the debris-free homocentric beam (7) of the short-wavelength radiation and a gas shutter preventing an exit of the protective gas through it.
 13. The source according to claim 1, wherein the permanent magnets (9) are located along an entire length of the casings.
 14. The source according to claim 1, wherein the means (4) for debris mitigation include foil plates (22) placed in each of the casings (6) and oriented in radial directions with respect to the plasma (2), substantially perpendicular to magnetic field lines.
 15. The source according to claim 1, wherein the plasma can be selected from a group consisting of: laser-produced plasma, z-pinch plasma, plasma focus, discharge produced plasma, laser-triggered discharge plasma.
 16. The source according to claim 1, wherein the plasma is a laser-produced plasma of a liquid metal target (17) supplied by a rotating target assembly (20) to a focus area of a laser beam (21).
 17. The source according to claim 16, wherein the target (17) is a molten metal layer, formed by centrifugal force on a facing to an axis of rotation (18) surface of an annular groove (19), implemented in the rotating target assembly (20).
 18. A method of collecting radiation, comprising: collecting by an optical collector a radiation emitted by plasma at a plasma formation location, and directing at least a portion of the emitted by plasma radiation to a focal point, wherein the emitted by plasma radiation is guided through at least two casings equipped by means for debris mitigating and arranged to form debris-free homocentric beams of the short-wave radiation coming out of casings to the optical collector.
 19. The method according to claim 18, wherein outside each casing permanent magnets creating a magnetic field inside the casings are used for mitigation charged fraction of debris particles and optionally include protective gas flow, foil trap, CNT membrane debris mitigation elements.
 20. The method according to claim 18, wherein the optical collector contains several mirrors installed in a path of each of the debris-free homocentric beams and reflecting surfaces of all mirrors lie on a surface of an ellipsoid or a modified ellipsoid, in one focus of which is the plasma. 